Dunedin researchers say a high-quality study has refuted claims fluoride in water can cause mental defects.

The study also backed other research indicating breastfeeding was associated with higher child IQ.

The University of Otago research was based on the Dunedin multidisciplinary study, that followed the health and development of about 1000 people born in Dunedin in 1972 and 1973 until they were aged 38.

Lead author Dr Jonathan Broadbent said the new research focused on subjects' fluoride exposure during the first five years of their lives - the critical period in brain development, after which IQ was known to be relatively stable.

The study compared the IQs of those who grew up in Dunedin suburbs with and without fluoridated water. Use of fluoride toothpaste and tablets was also taken into account.

IQ scores between the ages of 7-13 years and at age 38 were examined, as well as sub-test scores for verbal comprehension, perceptual reasoning, working memory and processing speed. Data on IQ was available for 992 and 942 study members in childhood and adulthood, respectively.

Researchers controlled for childhood factors associated with IQ variation, such as the socio-economic status of parents, birth weight and breastfeeding, and secondary and tertiary educational achievement, which was associated with adult IQ, Broadbent said.

"Our analysis showed no significant differences in IQ by fluoride exposure, even before controlling for the other factors that might influence scores," he said.

In line with other studies, we found breastfeeding was associated with higher child IQ, and this was regardless of whether children grew up in fluoridated or non-fluoridated areas."

Studies that fluoridation opponents had said showed fluoride in water could cause IQ deficits, and which they heavily relied on in council submissions and hearings around the country, had been reviewed and found to have used poor research methodology and a high risk of bias, Broadbent said.

"In comparison, the Dunedin Multidisciplinary Study is world-renowned for the quality of its data and rigour of its analysis," he said.

"Our findings will hopefully help to put another nail in the coffin of the complete canard that fluoridating water is somehow harmful to children's development. In reality, the total opposite is true, as it helps reduce the tooth decay blighting the childhood of far too many New Zealanders."

A report on the study was published in the American Journal of Public Health.